Study of the coupled thermo-mechanical constitutive model of clays is an important issue in several specific fields, especially in the analysis of nuclear waste isolation. Based on the results of undrained triaxial tests on Boom clay at different temperatures, a coupled thermo-mechanical elastoplastic damage model is established. The yield surface is based on that of the Drucker-Prager cap model. A hardening law and a thermal/mechanical damage evolution equation are presented. The developed model is implemented into ABAQUS finite element code through subroutine USDFLD. Model parameters of Boom clay are then calibrated from experimental results by back analysis. Three dimensional coupled THM modeling of ATLAS III in-situ heating tests in HADES at the HADES underground research facility is performed by the proposed constitutive model. The numerical results are in agreement with in situ measurements, demonstrating that the model can reasonably depict the main features of thermo-mechanical behaviors of Boom clay.